Total and Regional Fat-To-Muscle Mass Ratio in Relation to All-Cause and Cause-Specific Mortality in Men and Women

Context: Fat-to-muscle mass ratio (FMR) has been considered a potentially useful indicator for assessing disease risk. However, the association between FMR and risk of death remains inconclusive. Objective: This work aimed to investigate the associations of FMR (the whole body, trunk, legs, and arms) with all-cause and cause-specific mortality by sex. Methods: A total of 337 951 participants from the UK Biobank were included. Fat mass and muscle mass were estimated using a bioelectrical impedance assessment device. Cox proportional-hazard regression was applied to explore the associations. Results: During a median follow-up of 12.58 years, we recorded 22 391 (6.62%) deaths, of which 4427 were from cardiovascular disease, 11 740 from cancer, and 1458 from respiratory disease. We observed associations of different shapes (U-shaped for whole body and trunk, L-shaped for legs, and J-shaped for arms) between FMR and all-cause mortality (P for nonlinearity <.001). Compared with the lowest quintile of whole body and trunk FMR, the all-cause mortality risk was reduced by 12% to 14% and 7% to 15% in men and women in quintiles 2 to 4, respectively. The hazard ratio (HR) associated with arm FMR in men and leg FMR in men and women (quintile 5 vs 1) were 1.14 (95% CI, 1.05-1.23), 0.76 (0.71-0.82), and 0.78 (0.70-0.88) for all-cause mortality, respectively. The associations between FMR and cause-specific mortality were mostly similar to those for all-cause mortality. Conclusion: Our study revealed statistically significant associations of total and regional FMR with mortality, providing new evidence that FMR may be a potentially useful indicator for assessing mortality risk.